Easy! As a logged in member, you simply open the clue page and click the Change Status button on the right menu and complete the form. You can leave a short message for the planter. We'll try to send the planter an email to let them know.
You can see the list of the letterboxes that you've found in Letterboxes > Found Letterboxes. Sorry, but we don't have sort options for the list yet. You can always vote for the idea in the Ideas section.